A low platelet count (< 150 x 109/1) is seen in around 7-10% of pregnant women and may occur for a variety of reasons, ranging from the relatively benign (gestational thrombocytopenia) to the frankly sinister (HELLP syndrome).
Problems and special considerations
Conditions involving reduced platelet numbers or function include:
• Gestational thrombocytopenia (GT). This is the most common cause of low platelet count in pregnancy, accounting for 70% of all cases. It is seen in around 6% of parturients and manifests in late pregnancy. The reduction in platelet count is likely to be secondary to haemodilution and accelerated platelet destruction, and the level rarely drops below 70-75 x 109/l. In 75% of cases the condition is clinically unimportant, and in its mild form it may not need investigation. It does not appear to increase the risk of maternal or fetal haemorrhage and is usually not regarded as a contraindication to regional analgesia. Spontaneous resolution occurs within 6-8 weeks post-delivery, and the condition may recur in future pregnancies.
• Immune thrombocytopenia (ITP). This represents around 5% of the cases of thrombocytopenia in pregnancy and is seen in around 0.01-0.5% of pregnancies. It is characterised by the production of platelet autoantibodies that may cross the placenta, putting the fetus at risk of intracranial haemorrhage during delivery. The development of thrombocytopenia in early pregnancy or the existence of a pre-pregnancy low platelet count may favour the diagnosis of ITP over GT. Although the platelet count is often below 100 x 109/l, coagulation is rarely affected because the young platelets, which make up a higher than usual proportion of the platelet mass, are more aggressively haemostatic. Treatment is by corticosteroid therapy, administration of immunoglobulin (IgG), or a combination of both in severe cases, and this should be considered when the platelet count is below 20-30 x 109/l, in the presence of bleeding manifestations, or to achieve a platelet count of at least 50 x 109/l before delivery or obstetric interventions. Platelet transfusion may stimulate autoantibody production and is generally ineffective, but may be considered in combination with IgG in the presence of bleeding in severely thrombocytopenic parturients. Splenectomy may be considered in refractory cases.
A number of drugs may induce thrombocytopenia (Table 109.1).
• Thrombotic thrombocytopenic purpura (TTP) and haemolytic uraemic syndrome. Thrombotic thrombocytopenic purpura is a rare condition that, if untreated, may be associated with a maternal mortality rate of 90%. TTP manifests with haemolytic anaemia, thrombocytopenia and widespread vascular occlusion, often resulting in neurological disturbance and renal failure. A deficiency in von Willebrand factor cleaving protease has been identified as a causative factor. Unlike HELLP syndrome (haemolysis, elevated liver enzymes and low platelet count; see below), TTP usually presents in the second trimester. The high morbidity and mortality associated with this condition warrants aggressive intervention, and exchange transfusions and plasmapheresis may be employed. Haemolytic uraemic syndrome presents clinically in a similar way to TTP but the renal problems tend to be more severe.
Table 109.1 Drugs that may impair platelet function or cause thrombocytopenia
|
Impaired platelet function |
Thrombocytopenia |
|
Aspirin |
Heparin |
|
Non-steroidal anti-inflammatory drugs |
Thiazide diuretics |
|
Colloid plasma substitutes |
Hydralazine |
|
H2 blockers |
|
|
Digoxin |
|
|
Cocaine |
• Pre-eclampsia. This is accompanied by a low platelet count in about 20% of cases. Around 10% of women with severe pre-eclampsia will progress to develop its most malignant version, HELLP syndrome. Mothers with evidence of pre-eclampsia must have their platelet count monitored frequently, and a fall should be regarded as evidence that the condition is worsening. The thrombocytopenia of pre-eclampsia is often accompanied by clotting defects; therefore, regular coagulation tests should also be done. Management should be targeted at treatment of the underlying condition, ultimately by delivery, but specific therapy may be needed, including platelet transfusion and fresh frozen plasma in severe cases.
• Others. Reduced platelet concentration may also result from other causes of impaired production, such as bone marrow depression, viral infections, vitamin B12/folate deficiency, hereditary defects, paroxysmal nocturnal haemoglobinuria or alcohol toxicity; or from causes of shortened survival of platelets, such as malignancy, drugs (including heparin and methyldopa) or disseminated intravascular coagulation.
Management options
The obstetric anaesthetist is often called upon to make a decision regarding the advisability of regional analgesia and anaesthesia in these cases. In general, patients with a platelet count above 75 x 109/l in the absence of pre-eclampsia are unlikely to have significantly altered platelet function. Some authorities have even suggested that there is a very low risk of adverse outcomes following a neuraxial technique in women with platelet counts as low as 50 x 109/l. Current guidance suggests a stable platelet count of 75 x 109/l or higher as an acceptable cut-off point for the performance of regional anaesthesia in women with no risk factors or abnormal coagulation. The mother with a rapidly falling count should, however, be regarded with more suspicion than the one with a low, but stable, platelet level. In a patient with a platelet count below 75-80 x 109/l, the decision to perform a neuraxial technique should be taken by a senior anaesthetist and should be individualised, taking into account the underlying pathology, the general state of the patient (including any new bruising), and the underlying trend and rate of decline of the platelet count, with the risk of the procedure (epidural/spinal haematoma) balanced against the benefits (pain relief, better blood pressure control, avoidance of general anaesthesia).
Tests of platelet function, such as bleeding time, are very operator-dependent and therefore of limited predictive value. Commonly seen phenomena in pregnancy such as clumping of platelets or variability in platelet size, seen in GT and ITP, may cause underreading of the true count by automated devices. Genuine thrombocytopenia may be verified by examination of a peripheral blood smear, or by analysis of blood samples collected in tubes containing citrate anticoagulant rather than ethylene diamine tetra-acetic acid (EDTA). Routine coagulation studies are usually indicated in thrombocytopenia, in case any other defect should be present. The mother should always be questioned about excessive bruising or bleeding, since the presence of these may signify impaired platelet function in borderline cases.
Key points
• All patients with thrombocytopenia must be fully investigated.
• Trends in platelet count are more important than absolute values.
• There is no fixed cut-off point for the platelet count when regional analgesia is being considered.
• Patients should be asked about excessive bruising or bleeding.
Further reading
American College of Obstetricians and Gynecologists. Practice Bulletin No. 166: Thrombocytopenia in pregnancy. Obstet Gynecol 2016; 128: e43-53.
Association of Anaesthetists of Great Britain and Ireland; Obstetric Anaesthetists’ Association;
Regional Anaesthesia UK. Regional anaesthesia and patients with abnormalities of coagulation. Anaesthesia 2013; 68: 966-72.
Cines DB, Levine LD. Thrombocytopenia in pregnancy. Blood 2017; 130: 2271-7.
Myers B. Diagnosis and management of maternal thrombocytopenia in pregnancy. Br J Haematol 2012; 158: 3-15.